PDCD1 and cancer: Tauriello et al. found the inhibition of liver metastasis of colon cancer by transforming growth factor-β in mice, thus releasing cytotoxic T cells to respond to cancer cells to prevent metastasis, and thus, the use of TGF-β inhibitors to achieve immuno-osmosis is sufficient to increase the sensitivity to anti-PD-1/PD-L1 checkpoint therapy [103].